A machining technique frequently used commercially today to machine the main bearings and crankpin bearings on automotive crankshafts is known as "plunging". A single cutting tool insert is plunged or fed radially inwardly into the crankshaft at the bearing while the crankshaft is rotating. Plunging has several recognized disadvantages. Ordinarily there is a separate set up machining operation to mill a locating spot on the crankshaft. This is expensive and time consuming. Another disadvantage is the limited tool life of a single insert, resulting in a machine being down many times during a single eight hour shift. Stated differently, if only a single tool is used at each bearing, it will wear out rapidly and require frequent replacement, resulting in the entire machine being down many times during a single eight hour shift.
A still further disadvantage of plunging is that the machining forces are so great that special chucking is usually required to minimize radial deflection and prevent excessive permanent distortion of the crankshaft. A center drive chuck located about midway between the crankshaft ends can be used to minimize excessive distortion when both ends are machined simultaneously. Alternatively a transfer lathe has been used where one end of the crankshaft is machined first and then the crankshaft is turned end-to-end and rechucked to machine the other half. Even with these operations a follow-up green grind is normally used to correct for the permanent distortion during plunging.
It has been suggested that crankshaft bearings might be machined by broaching and, indeed, numerous broaching machines were proposed some forty to fifty years ago. It is not believed that these broaching machines have been commercially exploited in recent years. it might be surmised that the lack of commercialization of these early proposals was due to the relatively high cost of broaches and a failure to recognize the importance of distributing high machining forces required by broaching.